1. Center for Space Physics; Boston University; Boston MA 02215 USA
2. Department of Physics and Astronomy; University of Padova; I-35122 Padova Italy
3. Department of Electronic and Electrical Engineering; University of Bath; BA2 7AY United Kingdom
4. Institute for Complex Systems; ISC-CNR; 50019 Sesto Fiorentino Firenze Italy
5. William B. Hanson Center for Space Science; University of Texas; Dallas TX USA